Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever-increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products and especially, but not exclusively, portable products such as mobile phones, audio players, video players, PDAs, wearable devices, mobile computing platforms such as laptop computers or tablets and/or games devices, or devices operable in an Internet-of-Things (IoT) system or environment. Requirements of the mobile phone industry for example, are driving the components to become smaller with higher functionality and reduced cost. It is therefore desirable to integrate functions of electronic circuits together and combine them with transducer devices such as microphones and speakers.
Micro-electromechanical-system (MEMS) transducers, such as MEMS microphones are finding application in many of these devices. There is therefore also a continual drive to reduce the size and cost of MEMS devices.
Microphone devices formed using MEMS fabrication processes typically comprise one or more membranes with electrodes for read-out/drive deposited on the membranes and/or a substrate. In the case of MEMS pressure sensors and microphones, the read out is usually accomplished by measuring the capacitance between the electrodes.
To provide protection the MEMS transducer will typically be contained within a package. The package effectively encloses the MEMS transducer and can provide environmental protection and may also provide shielding for electromagnetic interference (EMI) or the like. For microphones and the like the package will typically have a sound port to allow transmission of sound waves to/from the transducer within the package and the transducer may be configured so that the flexible membrane is located between first and second volumes, i.e. spaces/cavities that may be filled with air (or some other fluid), and which are sized sufficiently so that the transducer provides the desired acoustic response. The sound port acoustically couples to a first volume on one side of the transducer membrane, which may sometimes be referred to as a front volume. The second volume, sometimes referred to as a back volume, on the other side of the one of more membranes is generally required to allow the membrane to move freely in response to incident sound or pressure waves, and this back volume may be substantially sealed (although it will be appreciated by one skilled in the art that for MEMS microphones and the like the first and second volumes may be connected by one or more flow paths, such as small holes in the membrane, that are configured so as present a relatively high acoustic impedance at the desired acoustic frequencies but which allow for low-frequency pressure equalisation between the two volumes to account for pressure differentials, for example due to temperature changes). It is noted that in some applications, such as bidirectional microphones, a sound port may also be acoustically coupled to the second volume, such that the membrane receives acoustic signal via both the front volume and the second volume.
FIG. 1a illustrates one conventional MEMS microphone package 100a. A MEMS transducer 101 is attached to a first surface of a package substrate 102. The MEMS transducer may typically be formed on a semiconductor die by known MEMS fabrication techniques. The package substrate 102 may be silicon or PCB or any other suitable material. A cover 103 is located over the transducer 101 attached to the first surface of the package substrate 102. The cover 103 may be a one piece structure, for example a metallic lid. An aperture 104, i.e. acoustic port, in the cover 103 provides the sound port and allows acoustic signals to enter the package. In this example the transducer 101 is wire bonded to the substrate 102 via terminal pads 105 on the package substrate 102 and transducer 101. As mentioned above, a sound port 104a may also be provided, for example in a bidirectional microphone, such that the membrane receives acoustic signals via both the front volume and the second volume.
FIG. 1b illustrates another known MEMS transducer package 100b. Again a transducer 101, which may be a MEMS microphone, is attached to the first surface of a package substrate 102. In this example the package also contains an integrated circuit 106. The integrated circuit 106 may be provided for operation with the transducer and may for example be a low-noise amplifier for amplifying the signal from a MEMS microphone. The integrated circuit 106 is electrically connected to electrodes of the transducer 101 and is also attached to the first surface of the package substrate 102. The integrated circuit 106 is bonded to the transducer 101 via wire-bonding. Alternatively, the integrated circuit 106 may be flip-chip bonded to the package substrate 102, or flip-chip mounted and wire bonded. A cover 107, for example a two-piece cap, is located on the package substrate so as to enclose the transducer 101 and the integrated circuit 106. In this package the cover 107 comprises an upper part or lid portion 107a and side walls 107b or a spacer region which are all formed, for example, from PCB. The cover 107 has a sound port 104 in the upper part 107a which allows acoustic signals to enter the package. As mentioned above, in a bidirectional microphone the package substrate 102 may also comprise a sound port 104a for allowing passage of acoustic signals to the transducer 101 from beneath the package substrate 102, i.e. for providing the bi-directional element.
FIG. 2a illustrates another MEMS transducer package 100c, for example a chip scale (CS) MEMS transducer package that is formed at wafer level, also known as a wafer level package (WLP). The MEMS transducer package 100c comprises a MEMS transducer device 109, which may be a MEMS microphone, bonded to a cap section 113. In this example the MEMS transducer device 109 comprises a substrate 110. Integrated electronic circuitry 112 is provided within the substrate 110, with a MEMS transducer 111, for example a flexible membrane, formed relative to a cavity in the substrate 110. The integrated electronic circuitry 112 may be provided for operation with the MEMS transducer 111 and may for example include a low-noise amplifier for amplifying the signal from a MEMS microphone. The integrated electronic circuitry 112 is electrically connected (not shown) to electrodes of the MEMS transducer 111, and is also electrically connected (not shown) to bonding structures 117 coupled to a first surface of the substrate 110. The substrate 110, MEMS transducer 111 and integrated electronic circuitry 112 effectively form a MEMS transducer device 109 comprised of an integrated circuit die, which may provide an analog or digital output.
This type of packaging arrangement may be referred to as a “bottom port” configuration, wherein the MEMS transducer package 110c is “flip-chip” bonded to the next level of interconnect when assembled in a host device, for example to a PCB within a consumer device.
In the package arrangement shown in FIG. 2a the bond arm length of the cap section 113 is illustrated as reference 114, with the bond arm length of the MEMS transducer device 109 (and in particular the substrate 110) illustrated as reference 115.
In the example of this type of bottom-port configuration there is no sound port within the cap section 113 itself. Instead, acoustic signals are channeled to the outer surface of the MEMS transducer 111 (i.e. the underside of the membrane in FIG. 2a) when the MEMS transducer package 100c is mounted during use, for example when mounted to a PCB or substrate within a consumer device. It is noted, however, that a bidirectional microphone may comprise a sound port 104a in the cap section 113 for allowing passage of acoustic signals to the transducer 111.
FIG. 2b shows a bottom-side view of the MEMS package 100c, illustrating how the integrated electronic circuitry 112 is formed to one side of the MEMS transducer 111, with a plurality of bonding structures 117 provided for electrical connection to another device, e.g. the consumer product in which the MEMS package 100c is assembled during use.
FIG. 2c shows a cross-section at an interface where the MEMS transducer device 109 and cap section 113 meet. As can be seen, in order to create a bottom port chip scale wafer level package for a MEMS capacitive microphone it is necessary to have a minimum width of substrate, e.g. silicon, around the complete interface between the substrate and the cap. The interface is where the substrate 110 and the cap 113 are “bonded” to one another, for example using an adhesive. Therefore, there is a minimum bond width (MBW) that is required if mechanical stability is to be achieved. The MBW, and hence mechanical stability, can depend on factors such as the height of the substrate and/or cap bond arms 115/114: the longer each or both arm the greater the MBW should be and vice-versa.
The dotted rectangle in FIG. 2b illustrates what the die size of a MEMS transducer substrate would typically be for a traditionally wire bonded MEMS microphone. The area outside this dotted rectangle (corresponding to the shaded area in FIG. 2c) therefore illustrates the additional silicon area that is required for bonding the MEMS transducer device 109 and cap section 113 together, i.e. the minimum bond width (MBW) area.
Therefore, because of the minimum bond width (MBW), the area of both the MEMS transducer device 109 and the cap section 113 needs to be greater than it would normally be in the case where a silicon substrate is traditionally wire bonded to a support substrate of a package. The net result is that, for both the substrate and cap wafers, the maximum number of die per wafer will be limited because of the extra area required for the MBW. Furthermore, the increased size of the MEMS transducer package 100c has the disadvantage of requiring more space in a host assembly, for example a greater footprint.